Function.prototype.mycall = function (context,...arg) {
  context = context || window
  //谁调用myCall
  const fn = Symbol('fn')
  context[fn] = this
  const res = context[fn](...arg)
  delete context[fn]
  return res
}

Function.prototype.mybind = function (context,...arg) {
  let _this = this
  return function(...arg2){
    _this.apply(context,arg.concat(arg2))
    return res 
  }
}
